@AmyEL I've always been on progesterone pessaries. However I believe some clinics use the Ovidrel as its hcg, to trick your body into thinking you are pregnant so the corpus luteum makes lots of progesterone from that. Then once your natural levels of hcg rise from pregnancy you don't need it anymore. I've seen some ladies have no luteal support at all with FET and get pregnant.
I'm doing HRT FET so because I won't ovulate I'll need support with progesterone until 12 weeks if I get pregnant, as there won't be a corpus luteum.